The present invention relates generally to chopper-stabilized amplifiers, and more particularly to a way of providing lower-cost operational amplifiers having both low 1/f noise and low drift.
In conventional chopper-stabilized operational amplifiers, the externally applied differential input signal is “chopped” so as to reverse its polarity in response to each pulse of the chopping signal before being applied to the input stage of the “core” amplifying circuitry. This causes a corresponding reversal of the polarity of the differential output signal produced by the amplifying circuitry. If the externally applied differential input signal is zero but the input transistors of the input stage are slightly mismatched so as to produce an input offset voltage, the chopping causes the resulting differential output signal to appear as pulses having a magnitude proportional to the input offset voltage. The differential output signal is “un-chopped” out of phase with the “chopping” of the externally applied differential input signal. This effectively cancels the effect of the input offset voltage, and also effectively cancels the effect of 1/f noise generated in the amplifier.
The closest prior art is believed to include                (1) ordinary chopper-stabilized operational amplifiers including input chopping switch circuitry coupled to the (+) and (−) inputs of an operational amplifier and output chopping switch circuitry coupled to the corresponding (+) and (−) outputs of the operational amplifier,        (2) an instrumentation amplifier having a structure similar to that shown in FIG. 1 except that operational amplifiers 2 and 3 are not chopper stabilized and instead the input signals Vin+ and Vin− are “chopped” or “swapped” ahead of operational amplifiers 2 and 3 and the output signals Vout+ and Vout− are “un-chopped” or “un-swapped”, and        (3) use of conventional gain boost amplifiers in a folded cascode stage of an operational amplifier as shown in Prior Art FIG. 7.        
The above-mentioned ordinary chopper-stabilized operational amplifiers provide low 1/f noise and low offset drift, but have the disadvantage that the output signals of the core amplifier, which may undergo large magnitude voltage swings (depending on the loop gain), must be un-chopped, i.e. “un-swapped”, at the chopping frequency. Therefore, the prior chopper-stabilized operational amplifiers have slow signal settling, and a large amount of power consumption is required to keep them operating properly at the chopping frequency. In the prior art operational amplifiers in which the input is “chopped” and output is “un-chopped”, if gain boost amplifiers are used they are normally located outside of the chopping loop. Also, most prior art chopper-stabilized operational amplifiers require more than two main amplifier stages, in some cases as many as five, to obtain an acceptably high open loop gain. (See U.S. Pat. No. 6,002,299 issued Dec. 14, 1999 to Thomsen.) This results in complex circuitry and high power consumption.
A simplified diagram of the above mentioned folded cascode circuit including a gain boost amplifier is shown in Prior Art FIG. 7, with the same or similar reference numerals to designate the same or similar elements that are shown in subsequently described FIGS. 2A-2C. A shortcoming of the gain boosted folded cascode circuit of Prior Art FIG. 7 is that the bias voltage VB3 applied to the gate of cascode transistor 49A needs to be very precisely controlled because (1) if VB3 is too high, the drain-to-source voltage VDS of transistor 50A will go into its triode region as the voltage on conductor 47A node voltage decreases, causing Vout to increase toward positive supply voltage VDD, (2) if VB3 is too low, the 1/f noise from the gain boost amplifier increases due to a decrease in the channel resistance of current source transistor 45, resulting in non-linear circuit operation, and (3) if VB3 is lowered more than a small amount, the 1/f noise is unacceptably increased. Therefore, the circuit of Prior Art FIG. 7 usually requires a trade-off to be made between the linearity of the operational amplifier and the 1/f noise. Therefore if the linearity is to be optimized, then the noise performance must be sacrificed.
Thus, there is an unmet need for an improved operational amplifier design which avoids the need to provide a very narrow range of bias voltages in a folded cascode circuit in order to provide both low 1/f noise and low offset drift.
There also is an unmet need for an improved chopper-stabilized operational amplifier having both low 1/f noise and low offset drift.
There also is an unmet need for an improved chopper-stabilized operational amplifier having both low 1/f noise and low offset drift, and also having relatively low power consumption.
There also is an unmet need for an improved chopper-stabilized operational amplifier having both low 1/f noise and low offset drift which requires only two main amplifier stages to achieve acceptably high open loop gain.